1. Field of the Invention
This invention relates to a hydraulic actuator, and more specifically to a hydraulic actuator suitable for reciprocating rotation or reciprocating straight line movement of an implement, such as a back-hoe attachment, provided on an earth-moving machine or construction machine.
2. Description of the Prior Art
Usually, implements such as a back-hoe attachment provided on an earth-moving or construction machine are operated by a reciprocably straight-moving or rotating hydraulic actuator. In order to stop such an implement smoothly at its stroke end or at any desired position in its stroke, it is desired to reduce impact by properly absorbing the inertial energy caused by its motion.
With a view to realizing this desire, hydraulic actuators equipped with a cushioning means were proposed. The cushioning means of the known hydraulic actuator comprises a cushioning port provided in each of two pressure oil chambers of the actuator, a line connecting each cushioning port to a reservior, and a relief valve disposed within each line. When the implement in such a conventional actuator approaches its stroke end and therefore, an output means of the actuator, such as a piston, approaches its stroke end, a main port in a pressure oil chamber on the discharge side (that is, the pressure oil chamber whose volume has descreased as a result of the movement of the output means) is closed to increase the pressure of the chamber on the discharge side. When this pressure increases beyond a prescribed pressure of a relief valve located in the line connecting this chamber to the reservior, the relief valve is opened, whereby the pressure oil present in this pressure oil chamber passes through the cushioning port and returns to the reservior through the relief valve. Upon the return of the pressure oil to the reservior, the pressure of the pressure oil chamber on the discharge side decreases below the prescribed pressure of the relief valve, this relief valve is again closed. When a control valve is switched over to its neutral position in order to stop the implement at a desired position in its stroke, and therefore, to stop the output means at a desired position in its stroke, the main ports of the two pressure oil chambers are closed. Thus, in the same manner as in the above example, the pressure of the pressure oil chamber on the discharge side increases to return the pressure oil therein to the reservoir through the cushioning port and the relief valve.
The cushioning means described above reduces impact by absorbing the inertial energy of the implement through the flow restricting action of the relief valve during the passing of pressure oil through the relief valve. However, this conventional cushioning means cannot perform a fully satisfactory cushioning. In other words, the conventional cushioning means cannot stop the implement smoothly as desired by fully absorbing the inertial energy of the implement, that is, the inertial energy of the output means. This is because the opening pressure and the closing pressure of a relief valve which restrict the flow of the pressure oil and absorbs the inertial energy are substantially the same, and equal the prescribed pressure of the relieve valve. Accordingly, the valve is closed at substantially the same pressure as the opening pressure, and even after the closing of the relief valve, the inertial energy remains and causes impact to the implement, and thus to the output means. Inertial energy can be fully absorbed at a substantially low pressure setting, but in turn, induced pressure oil would be more likely to escape from the oil chamber, resulting in deterioration of working efficiency. It is not wise therefore to determine the pressure of the relief valve at a value lower than a reted one.
There was also proposed a hydraulic actuator in which a cushioning port of a pressure oil chamber is connected to a reservior by as first line provided with a first relief valve and also to a control valve by a second line provided with a second relief valve and an orifice placed alongside the relief valve. When the implement (therefore, its output means) approaches its stroke end, a main port of a pressure oil chamber at the discharge end of the actuator is closed, and a pressure oil in the pressure oil chamber is returned through the second line and the control valve. When the flow of the pressure oil passing through the second line is restricted by the above second relief valve and the orifice, the inertial energy of the implement is absorbed. Since the pressure oil flows through the orifice in this hydraulic actuator even after the closing of the second relief valve, the inertial energy of the implement is absorbed almost satisfactorily at its stroke end. However, when the control valve is brought to its neutral position in order to stop the implement at a desired position in its stroke, the second relief valve and the orifice are in the closed state, and therefore, the pressure oil in the pressure oil chamber on the discharge side is returned to the reservoir through the first line. The flow of the pressure oil passing through the first line is restricted by the first relief valve in the same way in the case of the above-mentioned actuator thereby to absorb the inertial energy of the implement. Since the flow of the pressure oil passing through the first line is restricted only by the first relief valve whose closing and opening pressure are substantially equal to each other, this hydraulic actuator can neither absorb the inertial energy of the implement sufficiently when stopping the implement at a desired position in its stroke. When an orifice is provided alongside the first relief in the first line, the pressure oil also escapes when being introduced into the pressure oil chamber, thereby to cause pressure loss. It is evident therefore that an orifice cannot be provided in the first line. Furthermore, this hydraulic actuator requires two relief valves for each pressure oil chamber, and thus disadvantageously has a relatively complicated structure.